Public address system



A. B. SMITH PUBLIC ADDRESS SYSTEM Filed Nov. 12, 1936 mam Q30;

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Patented Mar. 7, 1939 PATENT OFFICE PUBLIC ADDRESS SYSTEM Arthur Bessey Smith, Evanston, 111., assignor to Associated Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application November 12, 1936, Serial No. 110,459

4 Claims.

I This invention relates in general to a public address system which is operated in combination with a telephone system.

The object of the present invention is the pro- 5 vision of an efficient and inexpensive address system operated in combination with a telephone system over the conductors included in the telephone cable without causing objectionable crosstalk.

The usual procedure in an address system is to provide a separate cable used exclusively for the address system. Howevenin many instances, for example in schools or the like, where an inexpensive address systemis desired, extra telephone cable conductors are usually available and may be used without the expense of providing a separate cable. i

In an address system where conductors for operating the loudspeakers are included in a telephone cable it is necessary to guard against crosstalk between the loud-speaker conductors and the telephone line conductors. Previously, two methods have been used to overcome this cross-talk. One is to shield the loudspeaker conductors which,

in known manner, effectively prevents crosstalk. The shielding of the loudspeaker conductor is expensive and therefore objectionable. The other method to reduce cross-talk to a point where it is no longer objectionable is to limit the transmission level used on the cable conductors of the address system to about the same value as the normal speech level. Heretofore, when the transmission level has been so limited, or reduced, it has been necessary to use an amplifier at the receiving or loudspeaker end'of the conductors. The use of an amplifier for each loudspeaker is expensive and therefore objectionable.

In the present invention it has been found that it is not necessary to shield the loudspeaker cable conductors in order to overcome objectionable cross-talk; nor is it necessary to provide an amplifier for each loudspeaker. It has been discovered by numerous tests that satisfactory operation of loudspeakers can be obtained over the 45 telephone cable conductors without producing objectionable cross-talk by providing permanent magnet type loudspeakers wound to an impedance equal to or less than 100 ohms at 1000 cycles per secondand by delivering not over one watt to 50 each loudspeaker which makes possible the use of the telephone cables without the need .of an amplifier for each loudspeaker.

The drawing discloses a single telephone cable having twisted pairs of wires used for both an 55 address system and for a telephone system. Al-

though only one telephone line ls'shown in the drawing, the majority of conductors in the cable are used for the telephone system which may be either an automaticor a manual system. The extra pairs of conductors in the cable are used 5 for the address system; three such address lines, commonly known as P. A. lines, being shown.

-It will be assumed, by way of an example, that a private automatic telephone system is installed in a school and that a telephone is installed in 10 each room. The telephone lines in the cable are run from the distributing frame DF, associated with the automatic switching equipment or switchboard, to the school rooms which are located on different floors of the school. The pub- 15 he address lines also included in the cable extend from the distributing frame D]? to the school rooms where such lines terminate in a receptacle for plug. connection with a loudspeaker or terminate directly in the loudspeaker. The public 20 address lines are jumpered at the distributing frame DF and extend to the control panel, preferably located in the principals room. The control panel comprises a switching key, such as SK--2; a balanced transformer, such as T-2, 5

- and a resistance, such as R2 for each P. A. line.

Each P. A. line is connected in multiple to the output transformer associated with a common amplifier which maybe used in connection with a radio, a phonograph or other-transmitting meansv 30 in the control panel.

The coils of the loudspeakers connected to the P. A. lines are each wound so as to have an impedance equal to or less than 100 ohms at 1000 cycles per second. In a similar manner the coils, of the balanced transformers connected to the address lines are each wound so as to have an impedance to equal that of the loudspeakers.

If desired a separate transformer in addition to the loudspeaker may be connected to P. A. line 40 at the speaker end and in this case the coil connected to the address line must have an impedance equal to or less than 100 ohms at 1000 cycles per second.

The resistances RI R2, and R9, are provided to equal the impedance of each address line, thereby providing dummy-l0ads which prevent distortion in the output of the transformer regardless of the number of P. A. lines connected to or disconnected from the output circuit.

Assuming now that the principal of the school desires to have one or more class rooms hear a certain broadcast which is being received on the radio in the control panel. The output of the radio is then connected to the amplifier and output transformer and the principal then operates the selecting keys corresponding to the classrooms, which, in his opinion, should hear the broadcast. For example, when key SK2 is operated the transformer T-2 is substituted for the dummyresistance R2 and the broadcast is then transmitted over the output circuit, through the make contacts of key SK-2, the transformer T-Z, and over the associated cable conductors to the loudspeaker in the corresponding classroom. Other P. A. lines are connected, as desired, by operating the corresponding selecting key.

Telephone conversation over the telephone lines in the cable and programs over the P. A. lines in the cable may be simultaneously transmitted without producing objectionable cross-talk because the impedances of the loudspeakers of the P. A. lines are equal to or less than 100 ohms at 1000 cycles per second. This arrangement of low impedance apparatus permits the transmission of relatively strong energy, in the neighborhood of one watt or less, in the audible range of frequency at a relatively low E. M. F. Tests have shown satisfactory operation of loudspeakers over these low impedance circuits without producing objectionable cross-talk within the limits universally accepted in commercial systems.

From the foregoing it will be seen that objectionable cross-talk is overcome when telephone lines and P. A. lines are included in the same cable without shielding the P. A. lines and without providing an amplifier for each loudspeaker.

What is claimed is:

1. The combination in an ordinary telephone cable, of telephone lines and public address lines, means including a circuit for transmitting a current having an audible range of frequency at a relatively low E. M. F. over said public address lines, and connections at each end of each public address line having an impedance equal to or less than 100 ohms at 1000 cycles per second for maintaining said current at a relative high energy of one watt or less and at a low E. M. F. thereby preventing objectionable cross-talk in the associated telephone lines in said cable.

2. The combination in a. telephone cable, of telephone lines and public address lines, a. loud--. speaker for each public address line, a coil associated with each loud speaker, one winding of each coil terminating each public address line, a transformer at the other end of each public address line having one Winding terminating each pled to each of said such line, means including a circuit for transmitting a current having high energy in the audible range of frequency at a low E. M. F. over said public address lines to operate said loud speaker, said transformer and loudspeaker windings connected to said public address lines having an impedance such as to maintain said current at a relative high energy of one watt or less and a low E. M. F. thereby preventing objectionable crosstalk in the associated telephone lines in the cable.

3. In combination, a communication cable including at least one telephone line and a public address line, an input transformer coupled to one end of said public address line, an output transformer coupled to the other end of said public address line, a signal translating device directly coupled to said output transformer, and means for impressing audio-frequency currents on said input transformer, the terminating'impedances of said input and output transformers being so proportioned relative to the impedance of said public address line that the power supplied to said translating device by way of said public address line is above the predetermined value required for satisfactory operation of said device and interference between the signals transmitted over said lines is maintained below an objectionable value.

4. In combination, a communication cable including a plurality of telephone lines and a plurality of public address lines, an input circuit coupublic address lines, a signal translating device associated with each of said public address lines, an output circuit coupled between each public address line and its associated signal translating device, a source of audiofrequency currents, means for selectively coupling said input circuits to said source, the terminating impedances of the input and output circuits associated with each of said public address lines being so proportioned relative to the impedance of the associated line that the power s pplied to the associated translating device is above the predetermined value required for satisfactory operation and interference between the signals transmitted over said lines is maintained below an objectionable value, and an impedance element associated with each input circuit for simulating the impedance of the associated input circuit when the said associated input circuit is not coupled to said source.

ARTHUR BESSEY SMITH. 

